The invention is directed to an improved press for rendering fatty bones in order to obtain rendered bones of low fat content suitable for the production therefrom of gelatin and also glue or animal meal.
Fatty bones are, for example, slaughter house or butcher's shop offal containing meat and fats besides bones. Loads of fatty bones supplied vary greatly and their content in soft substances (meat, fats, skins . . . ) can range between 0 and 40-50% according to their source, the mean content being about 15%. Due to their nature fatty bones supplied cannot be homogenized beforehand. They must be used as such. Fatty bones are usually treated in order to take off the major part of fats and non-collagenous proteinacious material and obtain the desired particle size range. Most often the bones are first sorted out for eliminating foreign substances such as pieces of iron or plastic material and are afterwards sent to a prebreaker and a breaker. The broken bones are heated by steam to a temperature of about 90.degree. to 95.degree. C. and charged in a press in order to recover a solid cake constituted in major part by bones and a liquid constituted by molten fats, water-soluble substances and in some cases, bone powder. The separated liquid is treated in order to recover fats and proteinous substances. The bones are dried and optionally treated for the production of gelatin.
It has been proposed to use multistage continuous screw presses. These presses are constituted by a cylindrical cage made out of wire netting, perforated plates or, most often longitudinally extending slats. In the cage or casing is located a drive arbor or tube on which is secured in each stage a continuous screw collar, vane or rib, with a thread configuration. The internal diameter of said screw collar increases in the direction of feed circulation of the treated bones and the outer diameter of the thread of the screw collar is constant and slightly less than the inside diameter of the cage so that the screw collar can freely rotate in the cage. Between two successive stages, there is disposed a threadless or transition zone formed by concentrically mounting on the drive tube two cylindrical sleeves or surface portions integral with one another. One of the sleeves is fixed to the continuous screw collar of the preceding stage and its outer diameter is equal to the largest inner diameter of the screw collar of that preceding stage. The other sleeve is fixed to the continuous screw collar of the next stage and its outer diameter is equal to the smallest inner diameter of the screw collar of that next stage. The driving tube also supports a threadless zone provided by a single cylindrical sleeve portion located at the outlet of the last stage of the press. The press may include two to six stages, but more often four. Such presses are described for example in U.S. Pat. Nos. 3,398,677 and 3,398,678.
These presses are currently used for rendering fish wastes, separating fat from cracklings in crude fats, extracting oil from oleaginous seeds such as soja, peanuts, etc. They can also be used for rendering slaughter house or butcher's offal containing a high proportion of soft substances (meat, fats, skins, etc.). But when treating fatty bones containing less than 20% of soft substances, the bones do not move forward regularly in the press, thus causing clogging with resultant jamming or breaking of the press.
In order to avoid such jamming or breaking, presses containing two driving tubes were proposed with the thought that the friction between the two bone cakes would induce the treated bones to move forward. However, as shown by test, such jammings continued with the result that the two drive tubes became twisted and were drawn aside out of alignment.
A rotating knife penetrating between the threads of the screw was also tried for it was believed that it would force the cake to move forward. However, the knife was often carried away by the excessively dense cakes, resulting in the knife being broken, or twisted or damaging the thread formation of the screw.